Saturation Recovery EPR

Abstract

Saturation recovery EPR measures electron spin relaxation times, Measurement techniques and applications to relaxation mechanisms, oximetry, Heisenberg exchange and spin-spin distance measurements are discussed. From early times, it has been noticed that the CW EPR spectra of some spin systems saturate at lower microwave powers than do other spin systems. There developed a qualitative and semi-quantitative understanding of ways to use these observations to characterize radicals, and in some cases to identify that a normally slowly-relaxing radical was in proximity to a faster-relaxing radical. The general understanding of the effects of a rapidly relaxing metal ion predates the EPR measurements. Recall, for example, that transition metal ions were added to some of the earliest NMR samples to shorten the proton relaxation times. Also, by the late 1950s it was understood that in solution broadened the lines of CW EPR spectra (see review by Hausser and Brunner, 1998). A qualitative understanding of relaxation times is essential for selection of parameters for CW EPR and ENDOR experiments and for prediction of feasibility of pulsed EPR experiments. Quantitatively measured relaxation times provide insight into electronic structure, motion, and other processes that contribute to relaxation. Early estimates of relaxation times were obtained from power saturation curves, however analysis of these data is complicated by the dependence of saturation on the product rather than on either or individually, and on spectral diffusion (Eaton and Eaton, 2000a). Most EPR signals in Abstract: